Influenza (flu for short hereinafter) is an acute respiratory infectious disease and harmful to human health, which is caused by the influenza virus and characterized by high prevalence, widespread, rapid propagation. Influenza virus can cause serious symptoms in the elderly and children with weaker immune systems, and immunocompromised patients, such as pneumonia or cardiopulmonary failure. Influenza virus was first discovered by Wilson Smith, a British, who called influenza virus as H1N1. The H denotes hemagglutinin; the N denotes neuraminidase, and the numbers represent different types. Influenza virus has caused the global pandemic for many times since the discovery, and the outbreak of influenza virus happens every decade or so, which causes enormous losses in worldwide. Influenza spreads around the world in a yearly outbreak, resulting in about 250,000 to 500,000 deaths, and about three to five million cases of severe illness, and a total of about 5% to 15% of people in worldwide are infected. Every time a pandemic was due to the emergence of new strains in humans. Usually, these new strains are caused by the spread of existing influenza virus from other animal species to humans.
Influenza viruses are RNA viruses belong to the family of orthomyxoviridae, which belong to the genus of influenza virus. According to the differences of the virion nucleoprotein (NP) and matrix protein (M) antigenic characteristics and genetic characteristics, influenza viruses are divided into three types: A, B and C. The three types of influenza viruses have similar biochemical and biological characteristics. The virus particle is 80-120 nanometers in diameter and usually roughly spherical, although filamentous forms can occur. Virus is constituted with three layers, and the inner layer is the viral nucleocapsid containing nucleoprotein (NP), P protein and RNA. NP is a soluble antigen (S antigen) with type specificity and antigenic stability. P protein (P1, P2, P3) may be polymerase required for RNA transcription and replication. Middle viral envelope consists of a lipoid layer and a layer of membrane protein (MP), MP has antigenic stability and type specificity. Outer layer is a radial tuber consisting of two different glycoprotein projections, i.e., hemagglutinin (H) and neuraminidase (N). H is a tool for viral absorption on sensitive cell surface which can cause agglutination of erythrocyte, N is a tool for breaking away from cell surface after the completing of virus replication, which is capable of hydrolyzing mucus protein and N-acetylneuraminic acid that locates at the end of cell surface specific glycoprotein receptor. H and N both have variation characteristics, and only have the strain specific antigen, the antibody of which has a protective effect.
Influenzavirus A has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics. The type A virus is the most virulent human pathogen among the three influenza types and cause the severest disease, and can be transmitted to other species and may then cause human influenza pandemic. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are: H1N1, which caused Spanish Flu in 1918; H2N2, which caused Asian Flu in 1957; H3N2, which caused Hong Kong Flu in 1968; H5N1, which caused pandemic threats in the influenza season of 2007-2008; H7N7, which has unusual zoonotic potential; H1N2, endemic in humans and pigs; H9N2; H7N2; H7N3; and H10N7.
Influenzavirus B has one species, influenza B virus, which causes local epidemic influenza and can not cause the global influenza pandemic. The only animals known to be susceptible to influenza B infection are humans and the seal. This type of influenza mutates at a rate 2-3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.
Influenzavirus C has one species, influenza C virus, which exists in sporadic form, and usually only causes mild disease in children. Influenzavirus C usually can not cause influenza pandemic, and infect humans and pigs.
Unusually for a virus, its genome is not a single piece of nucleic acid; instead, it contains seven or eight pieces of segmented negative-sense RNA. The genome of influenza A viruses encodes 11 proteins: hemagglutinin (H), neuraminidase (N), nucleoprotein (N), M1, M2, NS1, NS2 (NEP), PA, PB1 (polymerase basic 1), PB1-F2 and PB2. Hemagglutinin (H) and neuraminidase (N) are the two large glycoproteins on the outside of the viral particles. HA is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles. Thus, these proteins are targets for antiviral drugs. Furthermore, they are antigens to which antibodies can be raised. Influenza A viruses are classified into subtypes based on antibody responses to H and N. These different types of HA and NA form the basis of the H and N distinctions in, for example, H5N1.
Vaccination and usage of antiviral drugs are important tools for responding to influenza pandemic. Due to the high mutation rate of the flu virus antigen, the vaccine can't be produced in large scale before influenza pandemic. The two classes of antiviral drugs used against influenza are M2 protein inhibitors (amantadine and rimantadine) and neuraminidase inhibitors (oseltamivir, zanamivir, peramivir and Laninamivir). However, the influenza viruses have developed drug resistance to all these drugs. Therefore, continuing demand for new anti-influenza treatment agent is existing.
Favipiravir, a new antiviral agent, having a new mechanism, has been launched, which plays antiviral action by inhibiting influenza virus RNA polymerase to target the inhibition of viral gene replication, but the therapeutic effect and the drug resistance of influenza viruses still need to be proved. Therefore, other compounds as anti-influenza agents of this mechanism still needed to be researched.